Giancarlo Parenti

A Gene Network Regulating Lysosomal Biogenesis and Function

Master Controller Cellular organelles allow the localized regulation of specialized processes. Under certain conditions, such as increased growth, organelles may be required to alter their function. Coordinated regulation of the gene networks required for mitochondrial and endoplasmic reticulum function has been observed. Now, Sardiello et al. (p. 473; published online 25 June) have discovered a gene network regulating the lysosome, the major organelle involved in the degradation of internalized macromolecules. Many lysosomal genes were regulated by a single transcription factor, TFEB. TFEB itself can be activated when the lysosome malfunctions and can regulate both the abundance of lysosomes found in the cell, as well as the ability to degrade complex molecules, including a mutant protein that accumulates in patients with Huntingtons disease. These results may have implications for the treatment of human lysosomal storage disorders, which are characterized by the aberrant accumulation of macromolecules causing cellular dysfunction. Coordination of the genes that regulate lysosomal biogenesis occurs via a shared sequence motif and one transcription factor. Lysosomes are organelles central to degradation and recycling processes in animal cells. Whether lysosomal activity is coordinated to respond to cellular needs remains unclear. We found that most lysosomal genes exhibit coordinated transcriptional behavior and are regulated by the transcription factor EB (TFEB). Under aberrant lysosomal storage conditions, TFEB translocated from the cytoplasm to the nucleus, resulting in the activation of its target genes. TFEB overexpression in cultured cells induced lysosomal biogenesis and increased the degradation of complex molecules, such as glycosaminoglycans and the pathogenic protein that causes Huntington’s disease. Thus, a genetic program controls lysosomal biogenesis and function, providing a potential therapeutic target to enhance cellular clearing in lysosomal storage disorders and neurodegenerative diseases.

A cluster of sulfatase genes on Xp22.3: Mutations in chondrodysplasia punctata (CDPX) and implications for warfarin embryopathy

X-linked recessive chondrodysplasia punctata (CDPX) is a congenital defect of bone and cartilage development characterized by aberrant bone mineralization, severe underdevelopment of nasal cartilage, and distal phalangeal hypoplasia. A virtually identical phenotype is observed in the warfarin embryopathy, which is due to the teratogenic effects of coumarin derivatives during pregnancy. We have cloned the genomic region within Xp22.3 where the CDPX gene has been assigned and isolated three adjacent genes showing highly significant homology to the sulfatase gene family. Point mutations in one of these genes were identified in five patients with CDPX. Expression of this gene in COS cells resulted in a heat-labile arylsulfatase activity that is inhibited by warfarin. A deficiency of a heat-labile arylsulfatase activity was demonstrated in patients with deletions spanning the CDPX region. These data indicate that CDPX is caused by an inherited deficiency of a novel sulfatase and suggest that warfarin embryopathy might involve drug-induced inhibition of the same enzyme.

The sulfatase gene family.

During the past few years, molecular analyses have provided important insights into the biochemistry and genetics of the sulfatase family of enzymes, identifying the molecular bases of inherited diseases caused by sulfatase deficiencies. New members of the sulfatase gene family have been identified in man and other species using a genomic approach. These include the gene encoding arylsulfatase E, which is involved in X-linked recessive chondrodysplasia punctata, a disorder of cartilage and bone development. Another important breakthrough has been the discovery of the biochemical basis of multiple sulfatase deficiency, an autosomal recessive disorder characterized by a severe of all sulfatase activities. These discoveries, together with the resolution of the crystallographic structure of sulfatases, have improved our understanding of the function and evolution of this fascinating family of enzymes.

Transcription factor EB (TFEB) is a new therapeutic target for Pompe disease

A recently proposed therapeutic approach for lysosomal storage disorders (LSDs) relies upon the ability of transcription factor EB (TFEB) to stimulate autophagy and induce lysosomal exocytosis leading to cellular clearance. This approach is particularly attractive in glycogen storage disease type II [a severe metabolic myopathy, Pompe disease (PD)] as the currently available therapy, replacement of the missing enzyme acid alpha‐glucosidase, fails to reverse skeletal muscle pathology. PD, a paradigm for LSDs, is characterized by both lysosomal abnormality and dysfunctional autophagy. Here, we show that TFEB is a viable therapeutic target in PD: overexpression of TFEB in a new muscle cell culture system and in mouse models of the disease reduced glycogen load and lysosomal size, improved autophagosome processing, and alleviated excessive accumulation of autophagic vacuoles. Unexpectedly, the exocytosed vesicles were labelled with lysosomal and autophagosomal membrane markers, suggesting that TFEB induces exocytosis of autophagolysosomes. Furthermore, the effects of TFEB were almost abrogated in the setting of genetically suppressed autophagy, supporting the role of autophagy in TFEB‐mediated cellular clearance.

Lysosomal Storage Diseases: From Pathophysiology to Therapy

Lysosomal storage diseases are a group of rare, inborn, metabolic errors characterized by deficiencies in normal lysosomal function and by intralysosomal accumulation of undegraded substrates. The past 25 years have been characterized by remarkable progress in the treatment of these diseases and by the development of multiple therapeutic approaches. These approaches include strategies aimed at increasing the residual activity of a missing enzyme (enzyme replacement therapy, hematopoietic stem cell transplantation, pharmacological chaperone therapy and gene therapy) and approaches based on reducing the flux of substrates to lysosomes. As knowledge has improved about the pathophysiology of lysosomal storage diseases, novel targets for therapy have been identified, and innovative treatment approaches are being developed.

The Pharmacological Chaperone N-butyldeoxynojirimycin Enhances Enzyme Replacement Therapy in Pompe Disease Fibroblasts

In spite of the progress in the treatment of lysosomal storage diseases (LSDs), in some of these disorders the available therapies show limited efficacy and a need exists to identify novel therapeutic strategies. We studied the combination of enzyme replacement and enzyme enhancement by pharmacological chaperones in Pompe disease (PD), a metabolic myopathy caused by the deficiency of the lysosomal acid α-glucosidase. We showed that coincubation of Pompe fibroblasts with recombinant human α-glucosidase and the chaperone N-butyldeoxynojirimycin (NB-DNJ) resulted in more efficient correction of enzyme activity. The chaperone improved α-glucosidase delivery to lysosomes, enhanced enzyme maturation, and increased enzyme stability. Improved enzyme correction was also found in vivo in a mouse model of PD treated with coadministration of single infusions of recombinant human α-glucosidase and oral NB-DNJ. The enhancing effect of chaperones on recombinant enzymes was also observed in fibroblasts from another lysosomal disease, Fabry disease, treated with recombinant α-galactosidase A and the specific chaperone 1-deoxygalactonojirimycin (DGJ). These results have important clinical implications, as they demonstrate synergy between pharmacological chaperones and enzyme replacement. A synergistic effect of these treatments may result particularly useful in patients responding poorly to therapy and in tissues in which sufficient enzyme levels are difficult to obtain.

Pharmacological enhancement of mutated alpha-glucosidase activity in fibroblasts from patients with Pompe disease.

We investigated the use of pharmacological chaperones for the therapy of Pompe disease, a metabolic myopathy due to mutations of the gene encoding the lysosomal hydrolase α-glucosidase (GAA) and characterized by generalized glycogen storage in cardiac and skeletal muscle. We studied the effects of two imino sugars, deoxynojirimycin (DNJ) and N-butyldeoxynojirimycin (NB-DNJ), on residual GAA activity in fibroblasts from eight patients with different forms of Pompe disease (two classic infantile, two non-classic infantile onset, four late-onset forms), and with different mutations of the GAA gene. We demonstrated a significant increase of GAA activity (1.3-7.5-fold) after imino sugar treatment in fibroblasts from patients carrying the mutations L552P (three patients) and G549R (one patient). GAA enhancement was confirmed in HEK293T cells where the same mutations were overexpressed. No increase of GAA activity was observed for the other mutations. Western blot analysis showed that imino sugars increase the amount of mature GAA molecular forms. Immunofluorescence studies in HEK293T cells overexpressing the L552P mutation showed an improved trafficking of the mutant enzyme to lysosomes after imino sugar treatment. These results provide a rationale for an alternative treatment, other than enzyme replacement, to Pompe disease.

Lathosterolosis, a Novel Multiple-Malformation/Mental Retardation Syndrome Due to Deficiency of 3β-Hydroxysteroid-Δ5-Desaturase

We report the clinical, biochemical, and molecular characterization of a patient with a novel defect of cholesterol biosynthesis. This patient presented with a complex phenotype, including multiple congenital anomalies, mental retardation, and liver disease. In the patient’s plasma and cells, we found increased levels of lathosterol. The biosynthesis of cholesterol in the patient’s fibroblasts was defective, showing a block in the conversion of lathosterol into 7-dehydrocholesterol. The activity of 3β-hydroxysteroid-Δ5-desaturase (SC5D), the enzyme involved in this reaction, was deficient in the patient’s fibroblasts. Sequence analysis of the SC5D gene in the patient’s DNA, showing the presence of two missense mutations (R29Q and G211D), confirmed that the patient is affected by a novel defect of cholesterol biosynthesis.

Wilson Disease Protein ATP7B Utilizes Lysosomal Exocytosis to Maintain Copper Homeostasis

Summary Copper is an essential yet toxic metal and its overload causes Wilson disease, a disorder due to mutations in copper transporter ATP7B. To remove excess copper into the bile, ATP7B traffics toward canalicular area of hepatocytes. However, the trafficking mechanisms of ATP7B remain elusive. Here, we show that, in response to elevated copper, ATP7B moves from the Golgi to lysosomes and imports metal into their lumen. ATP7B enables lysosomes to undergo exocytosis through the interaction with p62 subunit of dynactin that allows lysosome translocation toward the canalicular pole of hepatocytes. Activation of lysosomal exocytosis stimulates copper clearance from the hepatocytes and rescues the most frequent Wilson-disease-causing ATP7B mutant to the appropriate functional site. Our findings indicate that lysosomes serve as an important intermediate in ATP7B trafficking, whereas lysosomal exocytosis operates as an integral process in copper excretion and hence can be targeted for therapeutic approaches to combat Wilson disease.

X-linked ichthyosis, due to steroid sulphatase deficiency, associated with Kallmann syndrome (hypogonadotropic hypogonadism and anosmia): linkage relationships with Xg and cloned DNA sequences from the distal short arm of the X chromosome

SummaryWe report a large Italian pedigree in which five out of six males are affected by a syndrome, following an X-linked inheritance pattern, characterized by ichthyosis, hypogonadotropic hypogonadism, and anosmia. The concurrence of features of X-linked ichthyosis (XLI) with those of Kallmann syndrome, another disease often inherited as an X-linked trait, prompted us to perform biochemical, cytogenetic, and molecular studies in relation to the short arm of the X chromosome (Xp). Steroid sulphatase (STS) activity was found to be completely deficient in all affected members of the family. Prometaphase chromosome analyses of two obligate heterozygous women and one affected male showed normal karyotypes. Xg blood group antigen analysis and molecular studies employing cloned DNA sequences from the distal segment of the Xp (probes RC8, 782, dic56, and M1A), did not provide evidence for deletions or rearrangements of the X chromosome. The linkage analysis showed no crossovers between the disease, Xg, and DXS 143, the locus defined by probe dic56, thus suggesting the possibility of a linkage between these two markers of the distal segment of Xp and the X-linked ichthyosis, hypogonadism, and anosmia syndrome.